An axial flow, gas turbine engine typically has a compression section, a combustion section, and a turbine section. An annular flow path for working medium gases extends axially through these sections of the engine. A stator assembly extends about the annular flow path for confining the working medium gases to the flow path and for directing the working medium gases along the flow path.
As the gases are passed along the flow path, the gases are pressurized in the compression section and burned with fuel in the combustion section to add energy to the gases. The hot, pressurized gases are expanded through the turbine section to produce useful work. A major portion of this work is used as output power, such as for driving a free turbine or developing thrust for an aircraft.
A remaining portion of the work generated by the turbine section is not used for output power. Instead, this portion of the work is used in the compression section of the engine to compress the working medium gases. The engine is provided with a rotor assembly for transferring this work from the turbine section to the compression section. The rotor assembly has arrays of rotor blades in the turbine section for receiving work from the working medium gases. The rotor blades have airfoils that extend outwardly across the working medium flow path and that are angled with respect to the approaching flow to receive work from the gases and to drive the rotor assembly about the axis of rotation. The stator assembly has arrays of stator vanes which extend inwardly across the working medium flow path between the arrays of rotor blades. The stator vanes direct the approaching flow to the rotor blades at a desired angle.
The stator assembly further includes an outer case and arrays of wall segments supported from the outer case which extend circumferentially about the working medium flow path. The wall segments are located adjacent to the working medium flow path for confining the working medium gases to the flow path. These wall segments have radial faces which are circumferentially spaced leaving a clearance gap C.sub.g therebetween. The clearance gap is provided to accommodate changes in diameter of the array of wall segments in response to operative conditions of the engine as the outer case is heated and expands or is cooled and contracts.
One example of an array of wall segments is an outer air seal. The outer air seal is part of the stator assembly of the engine and typically is formed of a plurality of arcuate seal segments. The outer air seal circumscribes the rotor blades to confine the working medium gases to the flow path. The stator assembly further includes an engine case, such as an outer case, and a support structure, such as an upstream support and a downstream support for supporting the seal segments of the outer air seal from the outer case. The seal segments are adapted by a pair of flanges to engages these supports. The outer case and the support structure position the seal segments in close proximity to the blades to block the leakage of the gases past the tips of the blades. The inwardly facing surfaces of the seal segments are commonly formed of an abradable material to enable the seal segments to accept rubbing contact with the tips of the rotor blades during operation. As a result of being disposed adjacent to the flow path, the surfaces of the segments and the segments themselves are in intimate contact with the hot working medium gases and receive heat from the gases. The segments are cooled to keep the temperature of the segments within acceptable limits.
One example of an outer air seal formed of segments is shown in U.S. Pat. No. 3,583,824 issued to Smuland et al. entitled "Temperature Controlled Shroud and Shroud Support". Smuland employs an outer air seal which is adapted by an upstream flange 44 and a downstream flange 46 to engage a support. Cooling air is flowed in a cavity which extends circumferentially about the outer air seal between the outer air seal and an engine case. A seal means, such as an impingement plate or baffle, extends circumferentially about the outer air seal to define an impingement air cavity 58 therebetween. A plurality of holes extend through the impingement plate to precisely meter and direct the flow of cooling air through the impingment plate across the compartment 58 and against the outer surface 59 of the seal segment. This cooling creates a large temperature gradient between the outer surface 59 and the surface of the abradable material adjacent. to the working medium flow path. The air is then gathered in the impingement air cavity. The cooling air is exhausted from the impingement air cavity through a plurality of axial passages 66 in the downstream hook 46 to provide a continuous flow of fluid through the plate and across the impingement cavity. This cooling air provides convective cooling to the edge region of the outer air seal as its passes through the compartment 64. Another example of a coolable outer air seal is shown in U.S. patent application Ser. No. 678,518 by Weidner executed on Nov. 30, 1984 entitled "Coolable Stator Assembly For a Rotary Machine" the material in which is herein incorporated by reference.
The abradable material on the outer air seal must accept the rubbing contact of the rotor blades without damaging the blades and without destructive results for the outer air seal. In addition, the abradable material must survive in the hostile environment of the turbine section of the engine. Representative abradable seal lands are shown in U.S. Pat. No. 3,817,719 to Schilke et al. entitled "High Temperature Abradable Material and Method of Preparing the Same"; U.S. Pat. No. 3,879,831 to Riggney et al. entitled "Nickle Base High Temperature Abradable Material"; U.S. Pat. No. 3,918,925 to McComas entitled "Abradable Seal"; and U.S. Pat. No. 3,936,656 to Middleton et al. entitled "Method of Affixing an Abradable Metallic Fiber Material to a Metal Substrate".
One attractive material for the abradable surface of the outer air seal is a ceramic facing material. Ceramic facing materials are desirable because of their compatability with the high temperature, hostile environment of the gas turbine engines. In addition, decreased amounts of cooling air are required to protect the seal structure which has a beneficial effect on engine performance. However, the durability of such structures are adversely effected by thermal cycling of the seal segment in the gas turbine engine which can cause cracking and spalling of the ceramic and even separation of the ceramic from the metal. Examples of improved seals having a ceramic facing surface with good resistance to thermal shock are shown in U.S. Pat. No. 4,289,446 entitled "Ceramic Faced Outer Air Seal For Gas Turbine Engines" issued to Wallace and U.S. Pat. No. 4,109,003 issued to Marscher entitled "Stress Relief of Metal Ceramic Gas Turbine Seals". Nevertheless, efforts are continuing to increase the durability of such seal segments to provide seal segments having an improved service life.
Accordingly, scientists and engineers are seeking to develop seal segments which employ ceramic material to form an abradable face, which have a substrate for carrying the ceramic material, and which are supported by conventional techniques in a gas turbine engine.